The present invention relates to a cathode ray tube having an improved cathode structure, and particularly to a cathode ray tube having an electron gun having a plurality of cathodes supported within respective eyelets of a glass-bonded cathode support assembly which is in turn fixed within and to a cup-shaped first grid electrode, and capable of retaining the insulation strength of the glass-bonded cathode support assembly for a long period of time.
Generally, color cathode ray tubes such as a color picture tube and a color display tube comprise a phosphor screen formed on an inner surface of a faceplate of a panel portion of an evacuated envelope, a shadow mask having a multiplicity of electron beam apertures and spaced from the phosphor screen within the panel portion, an electron gun of the in-line type housed within a neck portion of the evacuated envelope, and a deflection yoke mounted around a funnel portion of the evacuated envelope.
In the operation of the color cathode ray tube, three electron beams emitted from the electron gun are deflected by the deflection yoke and thereafter are projected onto picture elements of the corresponding colors of the phosphor screen through the electron beam apertures of the shadow mask to display a desired color image on the phosphor screen.
FIGS. 6A and 6B are constitutional views showing an example of the constitution of an electron gun used for a conventional color cathode ray tube, FIG. 6A being a side view thereof, and FIG. 6B being a top view thereof.
In FIGS. 6A and 6B, reference numeral 30 designates a cathode; 31 a cup-shaped first grid electrode (G1); 32 a second grid electrode (G2); 33 a third grid electrode (G3); 34 a fourth grid electrode (G4); 35 a fifth electrode (G5); 36 a sixth grid electrode (G6); 37 a shield cup; 38 multiform glass rods; 39 a stem; and 40 an electron gun. In this case, only one cathode 30 is shown, but actually there are three electron guns arranged in a line.
The cathodes 30, the cup-shaped first grid electrode 31, the second grid electrode 32, the third grid electrode 33, the fourth grid electrode 34, the fifth grid electrode 35, the sixth grid electrode 36, and the shield cup 37 are mounted, in order named from the stem 39, in spaced relationship between the pair of multiform glass rods 38 spaced from each other and arranged parallel with each other. This beaded assembly is supported on the stem 39.
The cathodes 30, the cup-shaped first grid electrode 31, the second grid electrode 32, the third grid electrode 33, the fourth grid electrode 34, the fifth grid electrode 35, the sixth grid electrode 36, and the shield cup 37 are supported on and secured to the pair of multiform glass rods 38 through electrode supports. The electron gun 40 is held in place within the neck portion in such a manner that the stem 39 mounting the electron gun 40 thereon is heat-sealed to the open end of the neck portion of the color cathode ray tube, and bulb spacer contacts 200 welded to the shield cup 37 hold the forward end of the electron gun 40 centered in the neck portion.
FIGS. 7A and 7B are a plan view and a sectional view, respectively, showing the constitution of a portion supporting cathodes of an electron gun 40 proposed in Japanese Patent Application Laid-Open No. 56-109429 Publication. Cathodes 30B, 30G and 30R composed of a cathode cap 150 provided with an electron-emissive surface, a cathode sleeve 151, and a skirt portion 152 are secured to an insulating substrate 120b by metal supports 170. The insulating substrate 120b is secured within the cup-shaped first grid portion 31 (FIG. 6A) by a metal member 110.
Normally, when the color cathode ray tube is operated for a long period of time, metal components are evaporated from cathodes 30G, 30B and 30R, and the evaporated metal deposit on the constituent parts arranged in proximity to the cathodes. It has been known that, where the cathodes 30G, 30B and 30R are, for example, oxide cathodes, evaporated metals are magnesium Mg from a cap-shaped base metal 150 containing Mg serving as a reducing agent, a chrome (Cr) from a metal sleeve 151. These metals deposit on the surface of the insulating substrate 120b with time of operation and deteriorate the insulation strength.
In FIG. 7B, in order to prevent the insulation strength of the electron gun from deteriorating, the top surface of the insulating substrate 120b is made slightly little higher than the upper open end of the metal member 110, and a step 130 is provided between the top surface of the insulating substrate 120b and the edge of the opening of the metal member.
With the above described constitution, even if the metals evaporated from the three cathodes 30G, 30B and 30R fall on the top surface of the insulating substrate 120b beyond the metal support 170 and contaminate the top surface of the substrate 120b with time of operation, the vertical part of the step 130 provided between the top surface of the substrate 120b and the upper open end of the metal member 110 is a shadow zone not irradiated by the evaporated metals such that the contamination by metals do not develop in the step 130, and the insulation strength is secured between the metal support 170 and the metal member 110.
Further, in the constitution described in Japanese Patent Application Laid-Open No. 56-109429 Publication, there is provided the following arrangement for preventing the deterioration of the electric insulation characteristics caused by the evaporation or sputtering of material constituting the cathode. In FIG. 7A, a line-of-sight passing through the upper end of the metal support 170 from the edge of the cathode cap 150 is designed so as to strike a point A as illustrated in FIG. 7A which is on the edge of the opening of the metal member 110 and is equidistant from the two adjacent cathodes 30B and 30G. The following is a relationship required in this case: EQU H=(L-LA).times.HA.div.(L-LB)
where
H is a protruding height of the metal support 170 beyond the insulating substrate 120b, EQU L={(S/2).sup.2 +LL.sup.2 }.sup.0.5 .apprxeq.S/2.sup.0.5, PA1 LL is a distance from the center of the cathodes 30B, 30G and 30R to the edge of the opening of the metal member 110, PA1 LA is a radius of the metal support 170, and PA1 LB is a radius of the cathode cap 150.
However, the invention described in Japanese Patent Application Laid-Open NO. 56-109429 Publication does not take into account a prevention of occurrence of a leakage path between the adjacent metal supports 170 on the insulating substrate 120b formed directly by the sputtering or evaporation from the two adjacent cathodes 30B and 30G.